The present invention relates to a method for the manufacture of sintered pressed pieces of iron reinforced by iron oxides.
A number of methods have heretofore been used to improve sintered pressed pieces containing iron. For example, such methods include, increasing the carbon content of the composition or adding alloys, such as alloys of copper, nickel, chromium, molybdenum, manganese and similar metals, or even the addition of microalloys of vanadium, titanium and similar metals. These additions may be made alone or in combination with a transformation strengthening, i.e., a strengthening caused by phase changes produced by heat treatment of the composition. In addition, dispersion agent and precipitation hardening of the composition were used. Suitable dispersion agents include oxides of aluminium, chromium, silicon, zirconium, cerium, yttrium, and thorium, and complex oxides of calcium-iron, magnesium-iron and others. Precipitation hardening is a reinforcement caused by precipitation of fine particles inside the grains. The resistance to movement by dislocation is thereby increased. Such technique, appears externally as an increase of the yield limit, which may be used as the measure of precipitation hardening. For improvement of quality of sintered materials, the importance of modern compacting methods, such as isostatic pressing, extrusion pressing, forging, rolling and the like have been employed.
For unalloyed sintered iron pressings, a typical strength range, depending on porosity, is 150 to 250 MPa for uncompacted and about 300 MPa for compacted pressings obtained, for instance, by forging. With use of alloys and heat treatment together with compacting at elevated temperatures, it is now possible to obtain materials having a strength of about 1000 MPa.
The microstructure of sintered unalloyed pressings without addition of iron oxides is formed by ferrite corns of a size of 15 to 30 .mu.m, with a porosity depending on the process of the pressings. In the case of pressing powdered iron with a pressure of 600 MPa, followed by its sintering at a temperature of 1100.degree. C. in a reduction atmosphere, a porosity of about 10 to 12 percent is achieved with pressings of a strength of only 180 to 200 MPa. Even with high pressure compacting at elevated temperatures, for instance by forging, this porosity can be reduced below 2 percent and a material with a corn size of 15 to 22 .mu.m and with a strength of only up to 300 MPa can be obtained.
It is an object of the present invention to provide an improved process for manufacturing improved sintered pieces from pressings.
It is a further object of the present invention to enable the manufacture, without use of alloys, of sintered uncompacted iron pressings with a strength up to 300 MPa and compacted pressings with a strength up to 700 MPa.